Electroluminescent light source



Dec. 19, 1961 M. WASSERMAN 3,014,149

ELECTROLUMINESCENT LIGHT SOURCE Filed Feb. 24, 1960 A LECTRIC'ALL) NON-LINEAR LAYER l2 ELECTROLUM/NESCENT LAYER I6 INVENTOR MOE WASSERMAN BY 1 a ATTORNEY tates 3,014,149 ELECTROLUMINESCENT LIGHT SOURC Moe Wasserman, Massapequa Park, N.Y., assignor to Sylvania Electric Products Inc, a corporation of Delaware Filed Feb. 24, 1960, Ser. No. 10,727 6 Claims. (Cl. 313-108) threshold response to applied voltages. A threshold response is required, for example, whenthe source is to produce light for an applied voltage which equals or exceeds a predetermined value and is to produce no light when the applied voltage falls below this value.

I have invented a new type of electroluminescent light source which has such a threshold response. In particular, my source has such a sharp threshold response that as the amplitude or magnitude of the applied voltage is increased by a factor of two for a specified level, the light output of my source can be increased by a factor substantially in excess of 10,000.

In accordance with the principles of my invention, my electroluminescent light sources comprise first, second and third electrodes, at least the third electrode permitting the passage of light therethrough. An electroluminescent layer is interposed between the second and third electrodes, and a layer of electrically non-linear material is interposed between the first and second electrodes. (This non-linear layer, in response to a voltage applied thereto, exhibits an electrical impedance which decreases as the voltage is increased.) The first electrode is smaller in area than the second electrode. The surface of the nonlinear layer is substantially equal to the surface area of the electroluminescent layer.

When an alternating voltage is applied between the first and third electrodes, it will be found that, at low voltage values, the impedance of the non-linear layer is high relative to that of the electroluminescent layer, and there will be substantially no light emitted from the electroluminescent layer. As the amplitude of the voltage is increased, the impedance of the non-linear layer, relative to that or" the electroluminescent layer decreases very rapidly. As a consequence, light is emitted from the electroluminescent layer, the total light emitted increasing very rapidly with relatively small increases in the amplitude of the applied voltage.

The electroluminescent layer, together with the second and third electrodes, acts electrically as a luminous capacitor. The nonlinear layer, together with the first and second electrodes, acts electrically as a network constituted by a non-linear resistor shunted by a capacitor.

The use of the first and second electrodes of differing areas permits the capacitance of the non-linear layer to be adjusted with respect to the capacitance of the elect oluminescent section of my source so as to produce the threshold effect described above. In the absence of the third electrode, 1 have found that the impedances of the non-linear layer and the electroluminescent layer cannot 3,014,149 Patented Dec. 19, 1961 ice be adjusted properly relative to each other and my source will not function in the manner desired. I

An illustrative embodiment of my invention will now be described with reference to the accompanying figure.

Referring to the figure, there is shown an electroluminescent device having a first electrode 10, a second electrode 14 and a third transparent electrode 18. An electrically non-linear layer 12 is interposed between electrodes 10 and 14. An electroluminescent layer 16 is interposed between electrodes 14 and 18. The entire structure including electrode 18 is supported on a glass substrate 20.

The electrode 18 can be formed from transparent tin oxide films. Electrodes 10 and 14 can be formed from the same material, or alternatively can be formed from an opaque metal, such as gold. The non-linear layer can be composed of cadmium sulfide particles (which are either free from copper or contain a small amount of copper), embedded in a glass enamel, as disclosed, for example, in my copending patent application Serial No. 827,111, filed July 14, 1959. The electroluminescent layer can comprise a dispersion of electroluminescent (copper activated zinc sulfide) particles also embedded in a glass enamel. Typically, the electroluminescent layer can be 1.5-2.5 mils thick. The non-linear layer typically can be about four times as thick as the electroluminescent layer. 1

As shown in the FIGURE, the area of the first electrode is smaller than the area of the second electrode. By adjusting the ratio of the area of the first electrode to that of the second electrode, the relative impedance values of the non-linear and electroluminescent portions of my device can be suitably adjusted in the manner previously indicated. Typically, this ratio can fall within the range 0.25-0.50.

When a 6000 c.p.s. voltage having a R.M.S. value of 250 volts was applied to a device as described above, the light output was found to be about 20 foot lamberts. As the R.M.S. value was reduced to volts, the light output dropped below 0.001 foot lamberts.

What is claimed is:

1. An electroluminescent device comprising first, second and third electrodes, the second electrode being interposed between the first and third electrodes, the third electrode permitting the passage of light therethrough, thearea of the first electrode being less than that of said second electrode; an electroluminescent layer interposed between said second and third electrodes; and an electrically non-linear layer interposed between said first and second electrodes, said non-linear layer consisting of cadmium sulfide particles embedded in a glass enamel, the surface areas of both of said layers being substantially equal.

2. An electroluminescent device comprising first, second and third electrodes, the second electrode being interposed between the first and third electrodes, the third electrode being transparent, the first and second electrodes being opaque, the area of the first electrode being less than that of said second electrode; an electroluminescent layer interposed between said second and third electrodestand an electrically non-linear layer interposed between said first and second electrodes, said non-linear layer consisting of cadmium sulfide particles embedded in a glass enamel, the surface areas of both of said layers being substantially equal.

3. An electroluminescent device comprising first, second and third transparent electrodes, the second electrode being interposed between the first and third electrodes, the area of the first electrode being less than that of said second electrode; an electroluminescent layer interposed between said second and third electrodes; and an electrically non-linear layer interposed between said first and second electrodes, said non-linear layer consisting of cadmium sulfide particles embedded in a glass enamel, the surface areas of both of said layers being substantially equal.

4. An electroluminescent device comprising first, second and third electrodes, the second electrode being interposed between the first and third electrodes, the third electrode permitting the passage of light therethrough, the area of the first electrode being less than that of said second electrode; an electroluminescent layer interposed between said second and third electrodes; and an electrically non-linear layer interposed between said first and second electrodes, said non-linear layer consisting of cadmium sulfide particles'embedded in a glass enamel, the surface areas of both of said layers being substantially equal, the ratio of the surface area of the first electrode to the surface area of the second electrode falling within the range 0.25-0.50..

5. An electroluminescent device comprising first, secnd and third electrodes, the second electrode being interposed between the first and third electrodes, the third electrode permitting the passage of light therethrough, the area of the first electrode being less than that of said second electrodes; an electroluminescent layer interposed between said second and third electrodes; and an electrically non-linear layer interposed between said first and second electrodes, said non-linear layer consisting of cadmium sulfide particles embedded in a glass enamel, the surface areas of both of said layers being substantially equal, the ratio of the surface area of the first electrode to the surface area of the second electrode falling within the range 0.25-0.50, the ratio of the thickness of the nonlinear layer to that of the electroluminescent layer being approximately 4:1.

6. An electroluminescent device comprising first, second and third electrodes, the second electrode being interposed between the first and third electrodes, the third electrode permitting the passage of light therethrough, the area of the first electrode being less than that of said second electrode; an electroluminescent layer interposed between said second and third electrodes; and an electrically noninear layer interposed between said firstand second electrodes, said non-linear layer consisting of cadmium sulfide particles embedded in a glass enamel.

References Cited in the file of this patent UNITED STATES PATENTS I 2,818,531 Peek Dec. 31, 1957 2,824,992 Bouchard Feb. 25, 1958 2,873,380 Kazan Feb. 10, 1959 2,875,380 Toulon Feb. 24, 1959 2,891,169 Nicoll June 16, 1959' 2,922,076 Sack J an. 19, 1960 

